Pyrolytic carbon is a material that has thus far been used to great satisfaction in the field of medical prostheses. One of the interesting properties of pyrolytic carbon is its Young's modulus is close to that of bone as well as its low coefficient of friction. As a result, pyrolytic carbon may work in frictional contact with bone without causing significant wear.
Yet pyrolytic carbon has the drawback of being poorly adapted as component of an assembly. On account of its brittleness it is difficult to attach pyrolytic carbon components using fasteners or welding. Nor can it be drilled as easily as other materials such as metals. For this reason, very few methods have been proposed thus far for joining a pyrolytic carbon component to another component in an assembly.
U.S. Pat. No. 5,458,647 describes an assembly that consists of a pyrolytic carbon cylinder force-fit into a bore in a metal component shorter than the cylinder. In this assembly, the pyrolytic carbon is permanently kept under stress. The danger exists, therefore, that the pyrolytic carbon will break when it is subjected to additional stresses during use. Moreover, the region of the junction between the component of the pyrolytic carbon cylinder engaged in the bore and the component situated outside of the bore constitutes a fragile region that may cause rupture of the pyrolytic carbon.
EP 1 365 165 discloses another type of assembly where a pyrolytic carbon component is shaped as a concave cone into which a convex cone of another component is fitted. One surface of the concave component perpendicular to the axis of the cones is abutting a corresponding surface of the convex component in order to account for the poor tensile strength of pyrolytic carbon, and thus limit the tensile stress of the concave component. This assembly also keeps the pyrolytic carbon permanently under stress. Moreover, the convex and concave cones must have a bearing surface sufficiently long for a good hold of the convex cone within the concave cone, as in all conical couplings. Such an assembly may thus not be realized when the pyrolytic carbon concave component has a small height.
U.S. Pat. No. 5,593,445 discloses a prosthesis with a pyrolytic carbon annular component snap-fit onto a spherical head of a convex component. The spherical head and the pyrolytic carbon annular component are engaged into a shell-shaped concave component. The convex and concave components are made of a titanium alloy or of ceramics. The annular pyrolytic carbon component is free to move on the spherical head but is held onto this head by the snap-fit arrangement. The convex and concave components are articulated like a ball-and-socket joint, and both are able to slide on the annular pyrolytic carbon component placed between them. This assembly of convex and concave components made of a material other than pyrolytic carbon and an intermediate mobile component made of pyrolytic carbon produces important friction, and thus risks of wear.